Periodic Table and Periodic Properties
Development of the table
| Scientist | Contribution |
|---|---|
| Dobereiner | Triads (groups of three elements with similar properties; middle element's mass is the average) |
| Newlands | Law of Octaves (every eighth element repeats), worked only up to calcium |
| Mendeleev (1869) | Arranged elements by increasing atomic mass; left gaps and predicted eka-aluminium (gallium), eka-silicon (germanium) and eka-boron (scandium) |
| Moseley (1913) | Showed that atomic number is the true basis |
Modern periodic law: properties of elements are a periodic function of their atomic numbers.
Structure of the modern table
- 18 groups (vertical columns) and 7 periods (horizontal rows).
- Elements in a group have the same number of valence electrons and similar properties. Elements in a period have the same number of shells.
- Blocks: s-block (groups 1, 2), p-block (groups 13 to 18), d-block (groups 3 to 12, transition metals), f-block (lanthanides and actinides, placed at the bottom).
- Number of elements in the periods: 2, 8, 8, 18, 18, 32, 32 (the first has 2).
- Total elements known: 118.
| Group | Name | Features |
|---|---|---|
| 1 | Alkali metals | Li, Na, K, Rb, Cs, Fr; very reactive; stored under kerosene |
| 2 | Alkaline earth metals | Be, Mg, Ca, Sr, Ba, Ra |
| 17 | Halogens | F, Cl, Br, I, At; salt-formers; very reactive non-metals |
| 18 | Noble (inert) gases | He, Ne, Ar, Kr, Xe, Rn; complete outermost shell, very unreactive |
- Hydrogen is placed in group 1 but is a non-metal.
- Metalloids (semi-metals): boron, silicon, germanium, arsenic, antimony, tellurium; between metals and non-metals; silicon and germanium are semiconductors.
Trends in the table
| Property | Across a period (left to right) | Down a group |
|---|---|---|
| Atomic size | Decreases | Increases |
| Metallic character | Decreases | Increases |
| Non-metallic character | Increases | Decreases |
| Electronegativity | Increases | Decreases |
| Ionisation energy | Increases | Decreases |
| Valency | Rises then falls (1 to 4, then 3 to 0) | Same |
- Most metallic (reactive) metal: francium (caesium is the most reactive stable one). Most non-metallic: fluorine, also the most electronegative.
- Smallest atom: hydrogen (helium by radius).
- Reactivity: metals become more reactive down a group; non-metals become less reactive down a group.
- Atoms lose or gain electrons to reach a noble gas configuration.
Some important elements
- Liquid at room temperature: mercury (metal), bromine (non-metal).
- Hardest natural substance: diamond (carbon). Best conductor of heat and electricity: silver. Lightest metal: lithium. Heaviest (densest) element: osmium. Lightest element: hydrogen. Highest melting point metal: tungsten. Most abundant gas in air: nitrogen. Most abundant element in the Sun: hydrogen.
- Radioactive elements: uranium, thorium, radium, polonium, radon.
- Noble gases: helium in balloons and airships (non-flammable); neon in advertising signs; argon in bulbs; xenon in flash lamps; radon is radioactive.
- Naming of elements, such as einsteinium, curium, mendelevium, nobelium, honour scientists.
Keep practising
Notes are only half the work. Test yourself with questions and discuss doubts with other aspirants.
Prepared for exam revision from the NCERT Science textbooks for Classes 6 to 10 and standard sources. Space missions, awards and records change, so check official sources for recent facts. Found a mistake? Tell us.